Doors are a piece of furniture used mainly to control the movement of dwarves, pets and liquids and define the exits of rooms. A door must be built next to a wall or other form of support. Doors appear using different glyphs depending on their material.

Forbid Passage: The door can not be passed unless it is permitted again or destroyed. Dwarves, animals, both wild and tame, and most invaders will all be unable to pass. However, thieves can pick the locks of forbidden doors, and building destroyers such as trolls will simply smash them down.

Keep tightly closed: Animals will not be able to pass through the door, but dwarves and invaders will not be hindered. (A dwarf that passes through the door can accidentally let in the animals... {Or say a goblin that just so happened to be one of the 50 goblins that ambushed you was to open this door and let out these 500 ravenous Wardogs.}) Because of pathfinding issues, this setting has been known to cause game lag.Bug:797

Set as internal: When designating rooms, a door set as internal will not block room expansion.

Doors can also be connected to levers or pressure plates to open on command. Doors also halt the movement of liquids (water and magma) in the manner of floodgates, and can be used in the same ways if a passageway is narrow enough.

The placement requirement for a door is that it must be positioned cardinally adjacent to a wall at the time of placement designation. That wall does not need to exist when the door is being constructed, nor does that wall need to exist for the door to continue to exist after it has been constructed. Thinking on this, one might discover that doors can be placed side by side in continuous strings, given a little extra effort.

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You will normally begin the process by anchoring your first door placement designation against a constructed wall. Immediately afterwards, you can use the designations menu to nRemove constructions to clear a space for the next door. Once the space is clear, you can designate the construction of a second door, even before the first door has been constructed. Repeat the process for your third, fourth, fifth, etc. door until you are satisfied. You can continue the process indefinitely as long as you possess doors and constructed walls to anchor your placement designations. Note that natural stone walls do not work this way; digging out a natural wall will deconstruct any adjacent no-longer-supported doors.

You can also use this strategy to unhook doors from walls and leave them as free standing structures. Why you would want to do this is up to you, but some players may find this useful for baiting Building destroyers away from your fortress.

Any item on the same tile as a door will cause the door to be propped open, which makes it impossible to lock and will allow fluids in. Such will also jam mechanically linked doors, needing to be cleared out and the lever or pressure plate triggered again to shut.

Doors destroy any fluid on their tile when they close.

If invaders move past a door, they will "take" control of the door and you will be unable to issue it qommands. If you can successfully move a squad through a door, it will be retaken, but the door will be returned in an unforbidden state. A door set to be tightly-closed will remain tightly closed when you retake it.

Doors are a vital, if somewhat double-edged, component of Fortress defense. You could build your own shogunate castle by replacing your walls with doors, a tactic which will disorient your enemies with unlimited pathfinding options and provide your Hammerdwarves with excellent ambushing potential. Be warned though, just like rice-paper walls have trouble keeping grown men at bay, you're going to have some fun with building destroyers. And thieves. Your haulers won't mind.

Doors block trade depot access, regardless of their settings. And no, you cannot build three-wide doors as a caravan garage door. You're better off using a bridge or linked floodgates for that.

All doors are watertight, but dwarfs may allow small amounts through when they open a door to pass. If you need doors to a watery area such as a reservoir for an artificial waterfall or flood trap, you can limit the mess from escaping water by making the exits "airlocks", two doors enclosing a one or two tile room between the tank and the main hallway. The small amount of water that escapes will be contained in the room and evaporate. If the water level can potentially get too high to evaporate, channeling a drain and placing grates or floor bars is even more effective.